In recent years, new energy-production techniques have attracted attention, from the standpoint of environmental issues, and among these techniques a fuel cell has attracted particular interest. The fuel cell converts chemical energy to electric energy through electrochemical reaction of hydrogen and oxygen, attaining high energy utilization efficiency. Therefore, extensive studies have been carried out on realization of fuel cells for civil use, industrial use, automobile use, etc.
Fuel cells are categorized in accordance with the type of employed electrolyte, and, among others, a phosphate type, a fused carbonate salt type, a solid oxide type, and a solid polymer type have been known. With regard to hydrogen sources, studies have been conducted on methanol; liquefied natural gas predominantly containing methane; city gas predominantly containing natural gas; a synthetic liquid fuel produced from natural gas serving as a feedstock; and petroleum-derived hydrocarbons such as petroleum-derived LPG, naphtha, and kerosene.
Upon use (e.g., civil use or automobile use) of fuel cells, the aforementioned petroleum-derived hydrocarbons, inter alia, kerosene, gas oil, and gasoline, are advantageously employed as hydrogen sources, since the hydrocarbons are in the form of liquid at ambient temperature and pressure, are easy to store and handle, and supply systems (e.g., gasoline stations and service stations) are well-furnished.
When hydrogen is produced from the petroleum-derived hydrocarbons, the hydrocarbons are generally autothermal-reformed, steam-reformed, or partial-oxidation-reformed, in the presence of a reforming catalyst. During such reforming processes, the aforementioned reforming catalyst is poisoned by sulfur content in the hydrocarbons. Thus, a key issue from the viewpoint of service life of the catalyst is that the hydrocarbons have been desulfurized to a low sulfur concentration.
When a commercial kerosene product is reformed so as to produce hydrogen for use in fuel cells, the sulfur content in the kerosene must be reduced to 0.2 mass ppm or lower, preferably 0.1 mass ppm or lower, over a long period of time, in order to prevent poisoning of the reforming catalyst by kerosene.
Among industrial methods for producing desulfurized kerosene, hydrodesulfurization is generally employed. In one embodiment of the method, hydrodesulfurization is performed by use of a hydrodesulfurization catalyst such as Co—Mo/alumina or Ni—Mo/alumina, and an H2S-adsorbent such as ZnO and at normal pressure to 5 MPa and 200 to 400° C. However, the method has drawbacks. Specifically, hydrogen must be recycled, thereby requiring a complicated facility for producing a fuel oil for use in fuel cells. In addition, the consumption of utilities increases. In a small-sized fuel cell system, employment of hydrodesulfurization for producing a fuel oil for use in the fuel cell renders the system very complicated. Therefore, a desulfurization system that does not require addition of hydrogen has been demanded.
Meanwhile, nickel-based or nickel-copper-based adsorbents (desulfurizing agents) are known to be desulfurizing agents which allow removal of sulfur content from a petroleum-derived hydrocarbon through adsorption under mild conditions without performing hydro-refining so as to lower the sulfur concentration to 0.2 mass ppm or lower (Japanese Patent Publication (kokoku) Nos. 6-65602, 7-115842, and 7-115843; Japanese Patent Application Laid-Open (kokai) Nos. 1-188405, 2-275701, 2-204301, 5-70780, 6-80972, 6-91173, and 6-228570 (nickel-based adsorbents); and Japanese Patent Application Laid-Open (kokai) No. 6-315628 (nickel-copper-based adsorbent)).
There have been proposed a variety of desulfurization conditions under which kerosene is desulfurized by use of the aforementioned nickel-based or nickel-copper-based desulfurizing agents. However, although the quality of kerosene serving as a feedstock varies depending on the production method, the relationship between the quality of kerosene serving as a feedstock and optimum desulfurization conditions has not yet been elucidated. Thus, the maximum performance of these desulfurizing agents has not yet been attained. In addition, detailed desulfurization conditions are not disclosed for the case where a gasoline fraction or a gas oil fraction is employed as a feedstock.
Among others, the aforementioned Japanese Patent Publication (kokoku) Nos. 6-65602 and 7-115842 disclose methods for desulfurizing kerosene employing a nickel-based desulfurizing agent without performing addition of hydrogen, and also disclose desulfurization reaction conditions. However, these documents never disclose in detail optimum desulfurization conditions that are determined as appropriate from the quality of kerosene.
The aforementioned Japanese Patent Application Laid-Open (kokai) No. 1-188405 also discloses a method for desulfurizing kerosene employing a nickel-based desulfurizing agent without performing-addition of hydrogen, as well as desulfurization reaction conditions. However, the reaction conditions disclosed therein are obtained by simply widening the range of the reaction conditions disclosed in the aforementioned Japanese Patent Publication (kokoku) No. 6-65602. Thus, similar to the above cases, the document never discloses in detail optimum desulfurization conditions that are determined as appropriate from the quality of kerosene.
The feedstock kerosene employed in the Examples of Japanese Patent Publication (kokoku) No. 6-65602 and that employed in the Examples of Japanese Patent Application Laid-Open (kokai) No. 1-188405 are almost identical in terms of quality, but differ in terms of optimum temperature range for desulfurization. This contradiction is considered to be attributable to the failure to elucidate the relationship between the quality of kerosene and optimum desulfurization conditions.
The aforementioned Japanese Patent Publication (kokoku) No. 7-115843 also discloses a method for desulfurizing kerosene employing a nickel-based desulfurizing agent without performing addition of hydrogen, as well as optimum ranges of desulfurization conditions. However, the patent publication includes the ranges of conditions disclosed in Japanese Patent Publication (kokoku) Nos. 6-65602 and 7-115842 and Japanese Patent Application Laid-Open (kokai) No. 1-188405, and includes no new information. Furthermore, the patent publication never discloses the relationship between the quality of kerosene and optimum desulfurization conditions.
In addition to the aforementioned patent publications, Japanese Patent Application Laid-Open (kokai) Nos. 2001-342466, 2001-342465, 2001-279274, 2001-279281, 2001-279260, 2001-279259, 2001-279257, 2001-279255, 2001-278602, 2001-276605, and 2001-252556, which were already filed by the present applicant, disclose desulfurization methods employing a nickel-based desulfurizing agent without performing addition of hydrogen, as well as desulfurization reaction conditions. However, no relationship between the quality of kerosene and optimum desulfurization conditions suited for the quality has been elucidated.